1. Field of the Invention
This invention relates to a bonding process involving hydroxide-catalyzed hydration/dehydration of substrate surfaces. This invention further relates to a highly reproducible, precision bonding method involving an alkaline hydroxide-ions-based bonding material. This invention still further relates to a bonding material composition; to a method of applying a bonding material to a surface; to a novel composite material; to a method of coating a substrate with a bonding material; and to a method of forming objects of a particular geometry from a bonding material.
2. Background of the Related Art
The bonding of materials is critical in making high performance instruments or devices. Depending on the particular application, the quality of a bonding method is judged on criteria such as bonding precision, mechanical strength, optical properties, thermal properties, chemical properties, and the simplicity of the bonding process. Three popular bonding methods of the prior art are optical contacting, epoxy bonding, and high temperature frit bonding. The salient features of each of these three prior art methods are summarized below.
Optical contacting is a room temperature process which employs no bonding material, and is thus suitable only for certain precision applications involving surfaces having reasonably good surface figure match. Ideally, if the bonding surfaces are thoroughly cleaned prior to bonding, the resulting interface will have low thermal noise and contain almost nothing susceptible to oxidation, photolysis, and/or pyrolysis. However, due to its sensitivity to surface particulate and chemical contamination (such as by air-borne contaminants) and other environmental factors (such as humidity), optical contacting produces bonds which are generally unreliable in strength. In addition, surface figure mismatch almost always exists to some extent. Consequently, strong chemical bonds rarely occur extensively across the interface, and voids are sometimes seen in the interface. Bonds produced by optical contacting do not consistently survive thermal shocks. Typically, optical contacting has a low first-try success rate. In case of failure, de-bonding usually degrades surface quality, and thus lowers success rate in re-bonding.
Epoxy bonding is usually a room temperature process and has a good success rate for regular room temperature applications. However, because epoxy bonding is typically organic based, the bonding is susceptible to pyrolysis (such as by high intensity lasers) and/or photolysis (such as by ultra-violet light) in high power density applications. The strength of the epoxy bond varies with temperature and chemical environment. Because the resulting wedge and thickness cannot always be precisely controlled, epoxy bonding is unsuitable for certain precision structural work. Epoxy bonding creates a relatively thick interface which makes optical index matching more of a concern in optical applications.
Frit bonding is a high-temperature process which creates a high-temperature rated interface. The interface is mechanically strong and chemically resistant in most applications. Because the frit material is physically thick and thus thermally noisy, it is unsuitable for precision structural work. For example, when optimized for bonding fused silica, frit bonding usually creates good coefficient of thermal expansion (CTE) matching with the bonded substrates at room temperature. The matching usually does not hold to a wider temperature range, however, resulting in strain and stress at or near the interface. Furthermore, a frit bond is opaque and inapplicable in transmission optics. Due to its high temperature requirement, frit bonding requires high temperature rated fixturing for alignment, and is thus expensive. Frit bonding is unsuitable if high temperature side effects, such as changes in the physical or chemical properties of the substrates, are of concern. Thus, each of the above prior art bonding methods has limitations and disadvantages.
U.S. Pat. No. 5,669,997 to Robbert et al. discloses a method of bonding optical or semiconductor members, in which grooves are formed in one of the surfaces to be bonded using high precision laser ablation. A low viscosity adhesive is provided in the grooves to allow chemical bonding of the members, essentially without the formation of an adhesive layer interface between the members.
U.S. Pat. No. 5,053,251 to Hara et al. discloses a method of repairing glass-lined equipment by a sol-gel process, the method including the repeated steps of applying a repair agent to a damaged area of the glass layer, and heating the repair agent for solidification and adherence to the glass. The ""251 also discloses an apparatus for heating the repair agent at the damaged area.
U.S. Pat. No. 5,143,275 also to Hara et al. discloses an improved method for repairing a glass layer of glass-lined equipment, in which a metallic fiber-containing sheet is disposed on the metal substrate of the damaged area, and welded to the metal substrate. A repairing agent may be supplied to the metallic sheet, and the damaged area heated to about 300xc2x0 F. to 350xc2x0 F.
U.S. Pat. No 3,007,832 to Milne discloses the sealing of joints between flexible sheets of alkali-soluble cellulosic material by applying a solution of alkali to the cellulosic material, and pressing the surfaces of the cellulosic material together. The incorporation of urea as a plasticizer in the sheet to be sealed allows superior sealing using lower concentrations (3-4%) of alkali.
U.S. Pat. No. 3,409,198 to Peterman discloses a bonding apparatus, including scanning means for detecting the presence on mating surfaces of surface roughness or contaminants which impede the bonding of the mating surfaces.
Japanese patent no. 3255-603-A to SONY Corp. discloses the junction of single-crystal ferrite and polycrystalline ferrite in which at least one hydroxide of K, Rb and Cs is formed at the junction interface of at least the polycrystalline ferrite.
Soviet Union/Russian reference SU 703-514 to RSFSR discloses a binder composition for building applications, comprising a mixture of 0.1 to 1.0% by weight of a metal sulphate (e.g., sodium sulphate), 1.0 to 10% by weight of alkali hydroxide (e.g., sodium hydroxide), and ground glass (the balance).
The present invention provides bonding methods and compositions which have numerous advantages over prior art methods and materials.
Accordingly, it is an object of the present invention to provide a method for producing bonds which are as precise and transparent as optical contact bonds and which also have the strength and reliability of frit bonds. It is a further object of the invention to provide such a bonding method which may be performed simply and inexpensively either at room temperature or over a broad temperature range.
One feature of the invention is that it provides an effective but simple and inexpensive precision bonding method. Another feature of the invention is that it provides a bonding method for substrate surfaces having surface figure mismatch. Another feature of the invention is that it provides a bonding method for substrate surfaces having good surface figure match. Another feature of the invention is that it provides a composite material including a bonding material. Another feature of the invention is that it provides a method of forming a composite material including a bonding material. Another feature of the invention is that it provides a method of coating a substrate surface with a bonding material.
One advantage of the invention is that it provides a reliable method for assembling Precision optical, optomechanical, and mechanical components. Another advantage of the invention is that it provides precision and non-precision bonding methods which can be performed under ambient conditions in air. Another advantage of the invention is that it provides a bonding interface which is thermally, optically, and electrically thin. Another advantage of the invention is that it provides a bonding interface which is resistant to organic solvents, and extremes of pH.
Another advantage of the invention is that it provides a bonding interface which is resistant to a powerful laser beam. Another advantage of the invention is that it provides a method for bonding a wide variety of substrate materials with a bonding material in which the substrate materials are insoluble. Another advantage of the invention is that it provides a method for forming an object having a specific composition and a defined geometry. Another advantage of the invention is that it provides a method of forming a multi-layered structure. Another advantage of the invention is that it provides a method for forming a wide range of composite materials.
These and other objects, advantages and features are accomplished by the provision of a method of assembling a system, including the steps of: a) providing first and second components having respective first and second surfaces to be bonded, wherein at least one of the first and second components is selected from the group consisting of optical components and optomechanical components; b) providing a bonding material including water and a source of hydroxide ions; c) applying the bonding material to at least one of the first and second surfaces; d) aligning the first and second components to form a bonding material interface between the first and second surfaces; and e) while maintaining alignment of the first and second components, curing the bonding material interface.
These and other objects, advantages and features are accomplished by the provision of a method of forming an object having a particular geometry, comprising the steps of: a) providing a bonding material comprising a silicate material and a source of hydroxide ions; b) providing a mold for the object to be formed, wherein the mold comprises at least one surface; and c) placing the bonding material on the at least one surface.
These and other objects, advantages and features are accomplished by the provision of a method of forming a composite material, including the steps of: a) providing at least one porous substrate material having a plurality of pores therein; b) applying a bonding material to the at least one porous substrate material, wherein the bonding material comprises a source of hydroxide ions; and c) curing the bonding material to form the composite material.
These and other objects, advantages and features are accomplished by the provision of a method of coating a substrate, including the steps of: a) providing a substrate; b) applying at least a first coating of bonding material to at least one surface of the substrate, wherein the bonding material comprises a source of hydroxide ions selected from the group consisting of ammonia water, calcium hydroxide, potassium hydroxide, sodium hydroxide, strontium hydroxide, sodium ethoxide, and sodium polymetaphosphate; and c) at least partially curing the at least a first coating of bonding material applied in step b).
According to one embodiment of the invention, there is provided a method for bonding a first surface to a second surface through hydroxide-catalyzed hydration and dehydration, in which hydroxide ions are applied to at least one of the surfaces and the first and second surfaces are then placed sufficiently close to each other to allow bonding between the first and second surfaces. The hydroxide ions are preferably contained in an alkaline aqueous solution which is applied to at least one of the surfaces. Materials which function as a source of hydroxide ions when in aqueous solution include: sodium hydroxide, potassium hydroxide, calcium hydroxide, strontium hydroxide, ammonia water, and sodium ethoxide. In certain applications, weaker alkaline chemicals (e.g., sodium polymetaphosphate) may be included as a source of hydroxide ions. Sodium polymetaphosphate serves to slow down the rate of hydroxide catalysis, as compared with use of, e.g., aqueous potassium hydroxide alone.
According to one embodiment of the invention, each surface to be bonded is preferably a surface of a material that can form a silicate-like network, and/or that can be chemically linked to a silicate-like network by means of hydroxide-catalyzed hydration and dehydration. When hydrated, both of these two categories of materials feature surface hydroxyl (xe2x80x94OH) groups. If one or both of the surfaces to be bonded do not meet either of the aforementioned chemical criteria, rough or porous surfaces may be physically adhered to a substrate by means of a silicate-like network. The silicate-like network may be formed either by the bonding material alone or by the bonding material together with a substrate surface to be bonded. As an example of physical adherence, an elongated, somewhat rigid silicate-like network may enter a pore on an adjacent substrate and act as an anchor via electrostatic forces, hydrogen bonds, and/or van der Waals"" bonds. Of course, in some cases, both chemical bonding and physical adherence may be involved at the same time.
The expression xe2x80x9csilicate-like networkxe2x80x9d refers to essentially a chemical-bond network similar to, but looser than, the bulk structure of silicon dioxide. In other words, the three-dimensional network is based on siloxane bridges (Sixe2x80x94Oxe2x80x94Si) with each silicon atom serving as a tetrahedral_center bonded to four oxygen atoms. However, the network is looser as compared with silicon dioxide, because it has more embedded and exposed silanol groups (Sixe2x80x94OH) and Sixe2x80x94Oxe2x88x92M+ groups (where M+ denotes a cation) as local terminating groups of the network.
Examples of materials capable of forming a silicate-like network by means of hydroxide-catalyzed hydration and dehydration include: silica (e.g., fused silica, fused quartz, natural quartz), silicon having a thermally-grown surface oxide layer, Zerodur(trademark), ULE(trademark), borosilicate, BK7 glass, SF5 glass, SK11 glass, opal, granite, and other silica-based or silica-containing materials, including certain laser crystals. Of these materials, silica generally forms silicate-like networks most efficiently.
Examples of materials which cannot themselves form, but can be linked to, a silicate-like network by means of hydroxide-catalyzed hydration and dehydration include:
i) metals and alloys: including aluminum, brass, copper, iron, nickel, niobium, mild steel, stainless steel, SXA foam, Stablcell(trademark), titanium, tungsten, zirconium;
ii) oxides of the above listed metals: including alumina, copper oxide, iron oxide, nickel oxide, niobium oxide, titanium oxide, zirconia; and
iii) crystals: including calcite (CaCO3), sapphire, Yttrium Aluminum Garnet (YAG, Y3Al5O12), and many other laser crystals.
The above list is not intended to be exhaustive. Additional materials that can be bonded according to the instant invention are listed hereinbelow.
According to the invention, each substrate surface may be cleaned prior to bonding. For bonding substrate material(s) that can form silicate-like networks in situ, if the surface figure match between the surfaces to be bonded is favorable, an alkaline solution containing a suitable concentration of hydroxide ions, but substantially or completely lacking silicate material, may be employed as the bonding material. For substrate materials that cannot generate silicate-like networks through hydroxide catalysis, or cannot generate silicate-like networks at a sufficient rate for a given surface figure mismatch, a silicate material may be included in the bonding material. Thus a bonding material used in the practice of the invention may include a source of hydroxide ions, and a silicate material. In either case, the bonding material may be described as a hydroxide-ions-based bonding material.
Regardless of whether substrate materials to be bonded are capable of forming silicate-like networks, the settling time (or settling period) in precision bonding of substrates having good surface figure match may be controlled by adjusting the concentration of hydroxide ions and silicate material in the bonding material. During the earlier portion of the settling time, surfaces which have been joined using bonding material according to the invention may be separated fairly easily. However, after the expiration of the settling period, attempted separation of bonded substrates, and re-bonding of the substrates, may be problematic. According to the invention, curing is normally incomplete after the expiration of the settling period.
If the surface figure match between the surfaces to be bonded is unfavorable, bonding coverage may be improved by including a filling material as a component of the alkaline hydroxide-ions-based bonding material. The filling material can be in the form of particulates, powders, foams, and/or a liquid. Such a filling material can facilitate bridging any interface gaps between substrate surfaces. Such a filling material preferably includes at least one material that can be hydrated to have exposed hydroxyl groups, and which can be chemically linked through hydroxide catalysis to a silicate-like network. The silicate-like network may be either generated in situ from the filling material and/or substrate material, or may be originally present in the bonding material. The composition of the bonding material may be adjusted to some extent in order to control the settling time. Advantageously, according to one embodiment of the invention, bonding may be performed at room temperature.
Bonding methods according to various embodiments of the invention are applicable to both precision and non-precision bonding. Precision bonding and non-precision bonding may be distinguished on the basis of alignment precision of substrates to be bonded in the direction approximately normal to the bonding interface. Precision bonding implies that the two substrates to be bonded make good direct contact with each other, and are in a bistability-free configuration. Non-precision bonding implies otherwise. In precision bonding, the bonding contact configuration is essentially determined by the surface figure match/mismatch of the substrates to be bonded, rather than by factors that interfere with the thickness of the interface. These latter factors include effects due to the nature of the bonding material, and the presence of contaminants in the interface.
In practice, precision bonding usually applies to substrates having substantially exact surface figure match. The presence of a filling material in the bonding material does not necessarily preclude precision bonding. In some situations, precision bonding may be performed on substrates lacking good surface figure match. In this case, precision bonding may begin with making good direct contact between the surfaces to be bonded prior to applying a bonding material. Again in this case, the bonding material may include filling materials, such as particulates and powders, which would interfere with bonding material thickness between substrates having good surface figure match.
Although there may be no clear-cut demarcation between precision and non-precision bonding, the following three types of bonding may be recognized, for convenience, under the invention:
(A) precision bonding using bonding materials that do not interfere with interface thickness;
(B) non-precision bonding using bonding materials that do not interfere with interface thickness (e.g., when precision is affected by contaminants and/or contact bistability);
(C) non-precision bonding using bonding materials that interfere with interface thickness.
The characteristics of bonds formed according to each of the above categories depend on factors such as surface figure match/mismatch, build-up of bonding material at the interface, as well as bonding coverage at the interface. Each of these factors are related to one another.
Category (B) is at least to some extent a quality control issue. Regarding categories (A) and (C), regardless of surface FIG. match/mismatch, methods of the invention provide 100% bonding coverage, and 100% fill factor is expected by default unless otherwise stated.
In view of the above, unless otherwise stated, the expression xe2x80x9cprecision bondingxe2x80x9d as used herein generally implies good surface figure match between surfaces to be bonded; and the expression xe2x80x9cnon-precisionxe2x80x9d bonding generally implies poor surface figure match.
In either precision or non-precision applications, various additives or property-modifying materials can be included in the bonding material in order to modify the physical and/or chemical properties of the bonding material, e.g., at the interface between the bonded substrates.
The above and other objects, advantages and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following, or may be learned from practice of the invention. The advantages of the invention may be realized and attained as particularly pointed out in the appended claims.